Act as a senior email marketing strategist with 10+ years of experience in optimizing transactional emails for [E-COMMERCE], [SAAS], and [FINANCIAL SERVICES] industries. Provide a detailed guide on best practices for designing, writing, and delivering high-conversion transactional emails. Include recommendations on subject lines, personalization, timing, mobile optimization, and compliance with [GDPR] or [CAN-SPAM] regulations. Highlight common pitfalls to avoid, such as overly promotional language or poor segmentation. Use real-world examples from [YOUR INDUSTRY] to illustrate key points. Ensure the response is actionable and backed by data-driven insights.